ESQL: union types with FORK leaks internal attributes in the output

[astefan]

Description

FROM apps, apps_short | EVAL x = id::integer | FORK (WHERE true) (WHERE true)

results in

      id       |     name      |    version    |$$id$converted_to$integer|       x       |     _fork     
---------------+---------------+---------------+-------------------------+---------------+---------------
null           |aaaaa          |1              |1                        |1              |fork1          
null           |bbbbb          |2.1            |2                        |2              |fork1          
null           |ccccc          |2.3.4          |3                        |3              |fork1          
...........

The important bit here is that the two ids from the two indices come from "incompatible" data types (integer and short) even if short should be regarded as perfectly convertible to an integer.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[astefan]

If the two datatypes that we can try to convert to are "compatible" the issue is not present.
For example, FROM sample_data, sample_data_ts_long | eval x = @timestamp::long | FORK (WHERE true) (WHERE true)